1. Field of the Invention
This invention relates to a high-voltage through-type ceramic capacitor, and more particularly to a high-voltage through-type ceramic capacitor for a magnetron used in an electronic range or oven or the like.
2. Description of the Prior Art
A high-voltage through-type ceramic capacitor has been generally used as a filter capacitor for a magnetron tube for an electronic oven or the like.
When an electronic oven is driven for a long period of time or subjected to a no-load test, heat generated from a heater circuit of a magnetron is transmitted through an aluminum stem and choke coils to central conductors of a capacitor. Recently, a magnetron tube is highly small-sized and correspondingly a filter box is small-sized. This causes a distance between the aluminum stem (heater side) and the central conductors of the capacitor to be reduced. Also, a measure to counter a fifth higher harmonic for satellite broadcasting causes the filter box to be free of any vacancy or hole to enhance a shielding effect of the filter box. Thus, an increase in temperature within the filter box is further promoted.
In general, a conventional high-voltage through-type ceramic capacitor includes an insulation case and an insulation cover, between which a ground fitment is fittedly arranged. In the insulation case is arranged a ceramic dielectric, of which earth electrode means is connected to the ground fitment. In the insulation case is poured an insulating resin material so as to surround the ceramic dielectric.
Now, such a conventional high-voltage through-type ceramic capacitor will be described more detailedly with reference to FIGS. 1 to 4.
A conventional high-voltage through-type ceramic capacitor which is generally designated by reference numeral 30 in FIGS. 1 to 4 includes a ceramic dielectric 32, which is formed with a pair of vertically extending through-holes 34 in a manner to be substantially parallel with each other. Also, the ceramic dielectric 32 is provided on an upper surface thereof with a pair of capacitor electrodes 36, which are separated from each other, and on a lower surface thereof with a common capacitor electrode 38. These separate electrodes 36 and common electrode 38 are formed with through-holes corresponding to the through-holes 34 of the ceramic dielectric 32, respectively. The capacitor 30 also includes a ground plate or fitment 40 formed at a central portion thereof with an opening 42 and provided on one surface thereof with an upstand 44 of a suitable height, which is arranged so as to surround the central opening 42. The ceramic dielectric 32 is fixed through the common electrode 38 on the upstand 44 of the ground fitment 40 using suitable means such as soldering or the like.
Further the through-type ceramic capacitor 30 includes a pair of central conductors 46 each including a conductor body 48 which is adapted to be covered with an insulation tube 50 formed of a suitable insulating material such as silicone. The insulation tubes 50 are inserted via the through-holes 34 of the ceramic dielectric 32 and the opening 42 of the ground fitment 40 and the conductor bodies 48 each are fittedly secured in an electrode connector or fitment 52 fixed on each of the separate electrodes 46 by soldering or the like.
The through-type ceramic capacitor 30 further includes an insulation case 54 securely fitted at a lower portion thereof on the upstand 44 of the ground fitment 40 so as to surround the ceramic dielectric 32 and an insulation cover 56 securely fitted at an upper end thereof in the upstand 44 of the ground fitment 40 so as to surround the central conductors 46. The insulation case 54 and insulation cover 56 are filled with insulation resin materials 58 and 60 such as epoxy resin or the like to cover an outside and inside of the ceramic dielectric with the resins or embed it therein, to thereby ensure insulation properties of the ceramic dielectric.
Each of the central conductors 46 further includes a fastening tab 62 integrally provided on an upper end of the conductor body 48 received in the insulation case 54, which is arranged in such a manner that it may be projected from an upper end of the insulation case 54 so as to facilitate connection of an external connector thereto.
The insulation case 54 and insulation cover 56 each are conventionally made of a polybutylene terephthalate (PBT) resin material.
The so-constructed high-voltage through-type ceramic capacitor is used in such a manner that the ground fitment 40 is mounted on a side wall of a filter box, resulting in the insulation cover 56 being positioned on an inside of the filter box. A temperature in the filter box is often increased to as high as 230.degree. C. during operation of an electronic oven for such purposes as described above. The insulation cover 56, as described above, is formed of a polybutylene terephthalate (PBT) resin material having a melting point of about 225.degree. to 228.degree. C. Accordingly, the insulation cover hangs down due to softening and/or melting by heat from the filter box to lead to contacting with a high voltage lead wire, resulting in an insulation failure.
Also, the ceramic capacitor tends to be small-sized corresponding to miniaturization of the magnetron tube and filter box described above. For this purpose, it is required to decrease dimensions of the insulation case. However, dimensions of the fastening tab and a receptacle fitted thereon are determined depending on a current capacity of a magnetron heater circuit, leading to a failure in miniaturization of the fastening tab and receptacle.
Now, a disadvantage due to a failure in miniaturization of the fastening tab and receptacle irrespective of miniaturization of the insulation case will be described with reference to FIG. 4. The failure results in an interval between the insulation case 54 and the central conductors 46 being substantially reduced. This, when a receptacle 64 is fitted on each of the fastening tabs 62, causes an outer wall 66 of the receptacle 64 to tend to strike against an upper end of the insulation case 54, resulting in fitting of the receptacle on the fastening tab 62 being often failed.
Also, miniaturization of only the insulation case 54 decreases a distance between the insulation case and the receptacles to lead to discharge between the receptacles 64 and the ground fitment 40, resulting in the burning and insulation failure of the components.
Further, it is desired that the insulation case 54 is made of a thermosetting resin material in view of its dielectric strength. Unfortunately, the thermosetting resin exhibits excessive adhesion to the insulating resin materials 58 and 60 poured in the case 54 to lead to a failure in exhibiting satisfactory dielectric strength. Thus, the conventional ceramic capacitor renders use of a thermosetting resin material for this purpose impossible.
Accordingly, it would be highly desirable to develop a high-voltage through-type ceramic capacitor which is capable of exhibiting satisfactory heat resistance sufficient to ensure good operability of the capacitor even under severe conditions and being positively small-sized while ensuring its satisfactory performance.